What, then, leads to the very different dust emitted by the long-period group of comets? One possible answer is that the presence of small grains in a comet may be an indicator of the evolutionary state of a comet, with smaller grains originating mainly from comets with highly evolved mantles and jet activity on their surface created after numerous perihelion passages. (Whether the small particles are produced in the violent emission associated with jets or are due to chemical differences in the jets is not clear, however.) After many passages, though, the reservoir of volatile material runs out, and the jets turn off with age.

The similarity in the dust albedo for 73P/SW-3, 2P/Encke [3], 55P/Tempel-Tuttle, and that found for the lP/Halley nuclear surface (pv -5% ; [5]), coupled with the preponderance of very large particles in the material emitted by the short period comets, suggest that the dust emitted is little modified from the that on the nuclear surface. I.e., large chunks of the comet's surface are being emitted slowly and relatively unchanged into the coma for the short period comets. In the other extreme of the dusty comets, we expect violent jet activity and/or the presence of ephemeral icy volatiles in the dust lead to changes in the emitted dust from that on the nuclear surface. Large increases in polarization and albedo in cometary jets, suggesting the production of excess small particles, has been found in the highly active comets Hale-Bopp [21] and Halley [22],

The evolutionary model also explains the increasing slope of the dust emission rate dependence with heliocentric distance for the more evolved short period comets (Figure 2), as the surface temperature has to be increased to higher values to drive off the denser mantle and/or scarcer volatiles in these comets. On the other hand, the large q short period comets, with surface temperatures unable to drive sublimation of the majority volatile H2O, age very slowly and are locked into a state similar to the long-period comets.

A test of this paradigm can be found in the transition-case Halley family comets like P/IRAS, which seems to emit small dust typical of the long period comets. Since this comet has a relatively short period of 13.2 years, which will, if anything, most likely decrease along with the semi-major axis of the orbit as it becomes increasingly locked into the 2:1 resonance of the typical Jupiter family comet. Thus future monitoring of the dust emission from P/IRAS should show a change from small- to large-particle dominated emission as the comet ages. Another useful test will be an extended 1- 300 ¡xm photometric imaging and 8-13 ¡xm spectroscopic survey of a large number ( > 50) of periodic and new comets using a single dedicated, well calibrated instrument, allowing us to improve the statistics and reduce the systematics of our search, such as will be possible with the upcoming SIRTF mission.

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